In chemical batteries such as lithium ion secondary batteries, between a positive electrode and a negative electrode, interposed is a separator having functions to electronically insulate between the positive electrode and the negative electrode and further to retain a non-aqueous electrolyte. Currently, in lithium ion secondary batteries, a microporous film comprising a polyolefin resin such as polyethylene and polypropylene is used as the separator. The microporous film is usually manufactured by drawing a sheet obtained by a molding method such as an extrusion.
However, in general, microporous films easily shrink under a high temperature, and there is a possibility for an expansion of a short circuit portion by the shrinkage, due to a short circuit reaction heat generated instantly at the time when an internal short circuit occurs or when a sharp-shaped matter penetrates through a battery. Such expansion of the short circuit portion generates further reaction heat, thereby accelerating an abnormal overheat of the battery. Further, when the battery is placed under a high temperature of 150° C. or more, the microporous film shrinks or melts, causing a possibility for a deformation of an electrode plate assembly (especially a wound type electrode plate assembly), a short circuit between the positive and negative electrodes, and an abnormal overheat.
In view of preventing an internal short circuit that occurs during a manufacturing process, in a non-aqueous electrolyte secondary battery having a negative electrode comprising a current collector and a negative electrode active material applied layer, a positive electrode comprising a current collector and a positive electrode active material applied layer, and a non-aqueous electrolyte, there has been proposed to form a porous protection film with a thickness of 0.1 to 200 μm on the surface of the negative electrode active material applied layer or the positive electrode active material applied layer. Herein, the porous protection film comprises a resin binder and a solid particle (patent document 1). Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 7-220759